Radiation pressure valves can be sealed with respect to the radiator neck by means of at least two sealing rings of which at least the inner one is a first O-ring, with the radiator neck comprising at least one radial outlet aperture located between the two sealing rings when the cap is completely closed. If the pressure relief or blow-off valve opens, cooling liquid and/or steam exits through the outlet aperture. If the complete pressure relief valve, meaning its sealing organ and its valve seat, is fastened to the radiator cap then the installation of the valve seat at the radiator neck is not required as far as the operability of this radiator cap is concerned. The same applies also to the vacuum relief valve. One can then install, check and if required adjust the pressure relief valve as well as the vacuum relief valve completely at the radiator cap in the workshop. The radiator cap must be sealed against the radiator neck at a suitable point so that, when the radiator cap has been put into its completely closed position, the cooling liquid or the steam formed therefrom cannot escape past the radiator cap between it and the radiator neck. On the other hand however a hydraulic connection towards the outside is necessary if the pressure relief valve opens, so that the cooling agent subjected to excessive pressure can exit from the neck. For this reason the inner seal with reference to the free radiator neck end must be arranged in such a way that the cooling liquid subjected to excessive pressure can basically only flow out through the pressure relief valve. On he other hand one must however assure that the cooling agent flowing out under excessive pressure can escape only through the outlet aperture of the radiator neck and not possibly along another travel path from the radiator neck. This requires the provision of the second sealing ring, wherein the outlet aperture of the radiator neck for the cooling agent flowing out under excessive pressure lies between these two seals. The second seal can possibly rest at the free radiator neck end, and be mounted to the inner face of a closing lid of the filler cap. In this case it can be a gasket. The inner seal in this type of construction is an O-ring retained at the radiator filler cap. Instead of the mentioned gasket a second O-ring can be fastened at the filler cap. It is even theoretically conceivable to provide in addition the mentioned gasket as a supplement to these two O-rings. At least in the normal case this gasket has no significance as far as a perfect sealing is concerned. It enters at the most into operation if the middle seal of these three seals fails.
The known filler cap is held upon the radiator neck by means of a threaded connection, and it comprises a cylindrical extension provided with a male thread, while the radiator neck is equipped with a matching female thread. The one or the several O-rings are located further inward in the neck as far as this thread is concerned.
It is also known to retain a radiator cap at a radiator neck by means of a bayonet thread, however in this case the radiator neck consists of metal at least at its end connected with the radiator cover cap. Apart from that no complete pressure relief valve is then present at the filler cap, rather only the valve head of same. The assigned valve seat is fastened at the metallic radiator neck, preferably it is molded therein. An accurate presetting of the pressure relief valve then becomes impossible in actual practice, because a radiator cap is normally supplied separately from the filler neck and it is therefore not known to begin with which specific filler cap will be used with which filler neck. In this type of construction an unfortunate clash of tolerances can arise.
Modern motor car engines operate at higher pressures and temperatures of the cooling medium. This necessitates tighter tolerances for the response pressures. This tolerance can be assured only if the pressure relief valve and the vacuum relief valve are completely located at the filler cap. On the other hand no possibility has been found so far of retaining such a filler cap at the radiator filler stub by means of a bayonet type lock.